I've come across something strange. When I download something with utorrent, it first downloads at full speed, but quick after the speed goes down to 0 and I can see that the SSD is reading something at 30 MBps, and then it starts with full speed again for a couple of seconds before it stops again.

But it doesnt look like it stalls, it goes full speed, and then down to 0, and when it does that i can see that the SSD is reading/writing data, and when that is done, the download goes full speed again..

But it doesnt look like it stalls, it goes full speed, and then down to 0, and when it does that i can see that the SSD is reading/writing data, and when that is done, the download goes full speed again..

Try a large direct download from somewhere if you really want to test. Torrents have so many factors affecting performance, it's difficult to say where the issue is.

But it doesnt look like it stalls, it goes full speed, and then down to 0, and when it does that i can see that the SSD is reading/writing data, and when that is done, the download goes full speed again..

A SLOW HDD can transfer data at ~100 megaBYTES per second. A FAST home internet connection can transfer data at 100 mega BITS per second, or roughly, 1/10th as fast. As has been said before, the Fusion drive is not the issue, it's your internet connection.

Seems like the fusion drive is doing something, and can't write data until it's done. But i dont understand what it is doing, if it was moving files, it would be writing data, now its just reading, nothing else and no activity on the HDD.

Seems like the fusion drive is doing something, and can't write data until it's done. But i dont understand what it is doing, if it was moving files, it would be writing data, now its just reading, nothing else and no activity on the HDD.

If you have only 79GB on your Fusion drive, ALL of your data resides on the SSD, so there is no file moving going on.
What app shows the information in your screen capture?

Do you think it's bad to use utorrent with Fusion Drive? Or shouldn't that be a problem?

Unless your machine is broken, Fusion isn't the issue. Once again, have you tried the option of telling uTorrent to download to an external drive? Tell it to download to a flash drive, or even an external hard drive. See if the problem continues when you download to something OTHER than the Fusion drive. It's basic problem solving... if uTorrent has no problem downloading to a drive other than your Fusion drive, your Fusion drive has a problem with it. If uTorrent does the same thing (start/stop) then it's a uTorrent problem.

Do you think it's bad to use utorrent with Fusion Drive? Or shouldn't that be a problem?

I use uTorrent with the fusion drive in my iMac and i have no such issue. I use a private tracker so my speeds are stable (i leave utorrent on 24/7) and very very fast The issue would be the utorrent setup or the tracker you are using.

You may make sure that you don't try do download illegal copies of Mac software, because that's the surest way of getting a trojan that messes your Mac up. And there are plenty of torrents for downloading illegal copies of music / videos / books etc., which is very damaging to the artists and may be damaging to you if you are caught.

But what makes you think that using an SSD drive would make any difference?

Nuke61 gave you the basic options of problem solving - if A and B together causes a problem, try to change either A or B and see if the problem stays or goes away.

Are you talking about the write limit on an SSD being smaller than that on a traditional hard drive?
If you are then there's little to worry about. It is true that SSD's have a more limited write limit, if you torrent normally (20-40GB a month), the other components of your computer will most likely go before your SSD does; heck your HD will probably go before your SSD. Just remember to backup daily.

Normal consumers like you or I don't need to worry about SSD write limits, it's mainly large companies with stacks of SSDs using them 24 / 7 writing loads and loads of small files to the SSD.

If you're torrenting 15-20GB a day, then it might be better practice to save it to a traditional hard drive then copy the whole file over when it's done.